Query on bias in Ultra-LD Mk2

The description of the bias compensation circuit for the
Ultra-LD Power Amplifier Mk.2 in the August 2008 issue states that a voltage of
70-100mV across the 0.1W emitter resistors gives a
quiescent current of 70-100mA per transistor. In fact the current would be
700mA-1A. This would in turn suggest that the amplifier was dissipating around
200W at idle.

Wouldn’t the actual voltage be 7-10mV? Furthermore, in the
absence of an adjustment, how can you guarantee that the quiescent current
settles at a level that is optimum for crossover distortion purposes?

Wouldn’t the actual bias current be highly dependent on the
characteristics of Q10 and Q11? Would a standard Vbe multiplier
arrangement where DQ12/DQ13 and DQ14/DQ15 were wired in parallel and Q10/Q11
were at ambient temperature be a better way to do this? Wouldn’t that guarantee
optimal thermal tracking? (F. Z., Barrack Heights, NSW).

The voltage across
the emitter resistors should be 7-10mV (not 70-100mV). This was corrected in the
September 2008 issue.

Also in September, we included details of how to vary the bias,
if necessary, because we have found that there can be a larger than expected
variation in the voltage drop across the internal diodes of the transistors.
This adjustment will cope with any variations in the Vbe of the drivers as
well.

We have not used a trimpot for this adjustment – we prefer to
do without trimpots if possible and we think that, in most cases, no adjustment
will be needed.

By the way, our circuit is not much different in principle from
the On Semiconductor application note AND8196/D which can be downloaded as a pdf
from the onsemi website: http://onsemi.com

In fact, the published bias circuit will always give much
tighter compensation than a Vbe multiplier which effectively monitors
the average of the Vbe drops (of driver and output transistors) at
"arms’ length" rather the very precise tracking made possible by the internal
diodes.

In any case, regardless of which bias stabilisation scheme is
used in a class-B amplifier, the driver transistors should be on the same
heatsink as the output transistors because any variation in the Vbe
of the driver transistors will affect the overall quiescent
current.

Using the speed control with shunt motors

I know the 12V-24V High Current Motor Speed Controller
(SILICON CHIP, March & April 2008) was
designed to handle permanent-magnet DC motors. What do you need to change or
modify in the circuit to allow the use of series-wound and shunt-wound motors?
(J. D., via email).